The configuration of patterns, for example recesses, in a silicon substrate using the plasma etching method is known. It is also known, for example for applications in micromechanics, to use fluorine compounds for anisotropic plasma etching. The fluorine radicals generated in the plasma act isotropically be with respect to silicon, however; i.e. the lateral etching rate corresponds substantially to the vertical one, which leads to correspondingly large mask undercuts and rounded profile shapes. In order to achieve a vertical sidewall with an etching method that uses fluorine compounds, precautions must additionally be taken in order to protect the sidewall selectively from etching attack, and to confine the etching to the pattern front, i.e. the bottom of the recess. Discrimination between the sidewall of the recess and the etching front is attained via a highly oriented vertical incidence of energetic ions, which are produced simultaneously in the plasma alongside the chemically active neutral radicals. The ions strike the surface of the substrate, the etching front being bombarded strongly by ions but the sidewalls of the recess, on the other hand, only relatively weakly. It is known to use polymer-forming gases such as CHF.sub.3, which are mixed directly with the fluorine-supplying etching gas, as the protective mechanism for the sidewalls. A polymer layer is deposited onto the sidewall from the polymer-forming monomers present in the plasma, while the fluorine radicals produced in the plasma at the same time etch the silicon substrate on the etching front, which is polymer-free because of the incidence of ions. It is disadvantageous that intensive recombination occurs, in the plasma and on the way to the substrate being etched, between unsaturated polymer-forming monomers and the fluorine radicals. To overcome this disadvantage, it is known to prevent the disruptive recombination of unsaturated polymer-forming monomers and the fluorine radicals capable of silicon etching by separating the plasma etching process into etching steps in which exclusively fluorine-supplying gases are used, and deposition steps in which exclusively deposition gases, such as the polymer-forming gases, are used. Because they are used in the plasma on a temporally separated basis, the two varieties of gas used do not encounter one another, so that appreciable recombination also cannot occur.
It is also known to passivate the sidewalls by using in the plasma, alongside the etching fluorine radicals, oxygen radicals or nitrogen radicals which convert the silicon of the sidewall at the surface into silicon oxide or silicon nitride, respectively. Since the dielectric surface is etched particularly strongly by the fluorine radicals with ion assistance and less strongly without ion assistance, etching proceeds essentially on the etching front, while the sidewall remains relatively protected. One substantial disadvantage of this method is that the silicon oxide or nitride layers generated at the surface are of only atomic thickness, i.e. are on the order of 1 nm thick or less. The silicon oxide or nitride layers generated at the surface therefore do not seal very well, and offer only incomplete protection. The result is that process control becomes more difficult, and that the process result is greatly influenced by secondary effects. The profile shapes of the patterns to be formed are never completely vertical, since sidewall attacks and thus also mask edge undercuts always occur. Cryogenic methods are used in order to increase the effectiveness of this passivation: cooling the silicon substrate to temperatures as low as -100.degree. C., in addition to oxygen or nitrogen passivation, "freezes out" the sidewall reaction. This method is described in U.S. Pat. No. 4,943,344. Disadvantages include the highly complex equipment and the costs associated therewith, as well as the comparatively poor reliability of the components.